The invention relates to a neutron detector.
Until recently most thermal neutron detectors were based on the use of proportional counters in which a gas is used which has a high cross section for the absorption of thermal neutrons. In the case of 10BF3 neutron capture leads to the ejection of an alpha particle and a lithium nucleus which have a combined kinetic energy of up to 2.8 MeV whilst 3He ejects less energetic alpha and triton particles having a combined kinetic energy of up to 0.8 MeV. These reaction products produce ionisation in the gas and the electrons are accelerated towards an anode wire. During this process, the ionisation charge is magnified and a signal is recorded in an associated amplifier every time that a neutron interacts in the gas. These proportional counters are typically cylindrical and may be made in lengths of up to 1 to 2 m and contain the gas at pressures of between 100 kPa and 2 MPa.
For some applications, these detectors can also be made so that an anode wire is arranged to form the diameter of a spherical gas enclosure. These spherical proportional counters, when enclosed within a substantial moderating enclosure, have found application in the construction of portable devices for monitoring the local intensity of neutron flux in the vicinity of a particle accelerator, nuclear reactor or fuel reprocessing plant. The design of these neutron-survey instruments or dosimeters has been described in the literature [1-4]. These devices are designed to have an omni-directional response. In particular, since the health hazard presented to those working in a neutron-flux environment is very dependent on the energy of the neutrons, there is a particular need for a neutron detector that, whilst having a broad-band energy response, generates an output proportional to the dose received by the operator. Such instruments commonly use a spherical proportional counter at the centre of the detector.
Recently, the world-wide scarcity of 3He and operational hazards associated with the deployment of systems based on the use of the noxious gas BF3 has prompted the search for alternative techniques for the detection of thermal neutrons. One possible candidate to fulfil this task is a design based on the use of 6Li nuclei. The lithium reaction leads to the generation of 2He++ and 3H+ fragments. These are released with a combined energy of 4.78 MeV. However, since there is no lithium based gas that can be used in a proportional counter, but this lithium reaction may be applied in a scintillation counter. There are a number of scintillation crystals containing Li atoms which could, in principle, be used. Alternatively, one can combine finely divided 6Li with a ZnS:Ag scintillation material to provide a large area detector. When combined with a suitable binder, layers of this mixture may be used to provide an efficient thermal neutron detector. Lithium atoms can also be combined in a scintillating glass to form either fibres or plates having good neutron detection efficiency.
Most of the applications in which neutron detectors are required, also have the requirement that they should be very insensitive to a high gamma-ray flux. Although LiI(Eu) crystals can be grown into boules having a diameter of approximately 30 mm, no attempt has yet been made to machine this material into a spherical form. Since a thickness of only 3 mm of LiI(Eu) crystals are required to fully absorb a thermal neutron and since the crystal is also highly hygroscopic, this material is not a good choice when replacing a spherical proportional counter for use as a portable neutron detector. When LiI(Eu) crystals are used in the form of a thin disc in contact with a photodetector, the detector can achieve a high gamma-ray rejection efficiency in small systems. In one known application, a small 4×4×4 mm cube of LiI(Eu) has been used at the centre of polyethylene spheres of different diameters [4]. By measuring the count-rate inside spheres of different diameters, the energy distribution of the incident neutrons can be inferred.
Two scintillation crystals that incorporate Li6 nuclei have been developed recently. They are commonly referred to as ‘CLYC’ [5] and ‘CLLB’. These materials are both hygroscopic but have other properties that make them attractive in that they could be used as the basis for a gamma-ray spectrometer and neutron detector. CLYC in particular has properties which make it especially useful for distinguishing between gamma-ray and neutrons by using pulse-shape discrimination techniques. Cerium activated lithium-glass scintillation material could in principle be used to fabricate a spherical scintillation counter for the detection of thermal neutrons. However, when used at the core of a survey instrument it would provide a less effective contribution to moderating the incident neutron flux. This is a disadvantage when seeking to design a compact, light-weight, detector system. Furthermore, cerium activated lithium-glass scintillation material provides a relatively poor gamma-ray rejection capability and it is also has a poor scintillation efficiency, that is, the number of optical photons/MeV.
Therefore there is a need to produce a more compact, light-weight neutron detector that provides an improved gamma-ray rejection capability.